A turbocharger is used in automotive vehicles to enhance the combustion engine performance by blowing compressed air into the cylinders of the engine.
A turbocharger generally comprises a housing and a shaft extending through an opening formed on the housing. A turbine wheel is mounted on a first end portion of the shaft and located in an exhaust gases passage of the combustion engine. A compressor wheel is mounted on an opposite second end portion of the shaft and located in an admission gases passage of the engine. Turbocharger further comprises rolling bearings disposed between the shaft and housing.
Rolling bearings are preferably located at each end of shaft. Rolling bearings generally comprise each a stationary outer ring fixed in housing, a rotating inner ring fixed with rotating shaft, a set of balls radially arranged between the rings, and a cage that circumferentially maintain the balls. Cage is annular and comprises a plurality of through holes forming pockets wherein balls are housed. Pockets are axially delimited between annular heels so as to firmly axially maintain the balls during high rotation speed from 50 000 rpm up to 350 000 rpm.
When the turbine wheel is rotated by the flow of the exhaust gases, the shaft and the compressor wheel are rotated which leads to a compression of the admission gases introduced into the cylinders of the combustion engine.
Turbocharger works at very high speed and lead to important temperatures within, in particular in bearings. It is therefore necessary to efficiently lubricate the bearings to avoid any extra heating and failure.
It is known from U.S. Pat. No. 5,076,766 to provide lubrication ducts to the stationary outer ring so that lubricant can be supplied in the radial space defined between the outer and inner rings. Advantageously, lubrication duct extend along an injection direction towards the balls of rolling bearings. In EP-B1-2657553 and US-A1-2008/267548, inner rings further comprise annular grooves with inclined wall so as to direct the injected lubricant towards the balls.
However, the cage heel oriented towards the interior of turbocharger forms a bottle neck for lubricant. The radial space between inner and outer rings is strongly reduced by the annular cage heel, even with an annular radial groove on inner ring that extend within the cage heel. The blocking effect is increased by the high rotation speed of cage.